Fuel casting apparatus with collapsible core



April 1 4, 1970 N. H. KATZ ETAL 3,506,235

FUEL CASTING APPARATUS WITH COLLAPSIBLE CORE 2 Sheets-Shet 1 Filed Sept.12, 1967 INVENTORS NORMAN H. KATZ ROGER N. BEAM ATTORNEY FUEL CASTINGAPPARATUS wi'ra COLLAPSIBLE CORE Filed Sept. 12, 1967 April 14, 1970 N.H. KATZ ET AL 2 Sheets-Sheet 2 INVENTORS NORMAN H KATZ ROGER N BEAMATTORNEY United States Patent 3,506,235 FUEL CASTIN G APPARATUS WITHCOLLAPSIBLE CORE Norman H. Katz, Northridge, and Roger N. Beam,

Chatsworth, Calif., assignors, by mesne assignments, to the UnitedStates of America as represented by the United States Atomic EnergyCommission Filed Sept. 12, 1967, Ser. No. 667,638 Int. Cl. B29c 11/00US. Cl. 249-82 3 Claims ABSTRACT OF THE DISCLOSURE Apparatus disclosureand method for casting annular uranium carbide and other nuclear fuelelements. The mold includes a solid graphite rod within an annulargraphite core. The core is designed to collapse upon cooling of themolded fuel, while the graphite rod serves as a heat sink for the moldto prevent premature destruction of the core. The core and rod in oneembodiment are separated by an air gap clearance sufi'icient to permitthe molded fuel to contract when cooling without destroying the hollowcore. In another embodiment, the solid rod is sectioned and includesgraphite spacer rings for permitting expansion and contraction of therod sections to thereby provide intimate thermal contact between coreand rod when the fuel is molten, while per mitting contraction of therod segments as the fuel cools.

BACKGROUND OF THE INVENTION The invention relates to casting annularshaped fissile fuel such as uranium carbide or oxide. More particularly,the invention relates to a particular method and apparatus for suchcasting. This invention arose in the course of, or under, ContractAT(381)430 with the United States Atomic Energy Commission.

Uranium carbide is a well known fuel element for nuclear reactors. Onemethod for producing such fuel elements is by casting. Normally, suchfuel elements are in the form of rods so that casting presents noparticular difiiculty.

The casting of annular fuel elements has been difficult because of highcoefiicient of thermal expansion and the inherent brittleness of uraniumcarbide. These characteristics limit both the amount of machining whichmay be performed on such cast elements and limit the obtaining of anycontrolled dimensions for the casting.

When such elements have been cast using solid graphite cores,substantial cracks frequently develop in the uranium carbide during thecooling process because of the differential expansion between the castUC and solid graphite core.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide substantially crack-free cast UC.

Another object is to provide cast UC having controllable dimensions.

A further object is to provide improved annular fissile fuel elements.

The invention provides these and other objects by using 3,506,235Patented Apr. 14, 1970 a molding process including an annular graphitecore enclosing a solid core both within a mold casing, the annular corebeing designed to collapse when the molded metal cools and the solidcore serving as a heat sink during the cooling process. 'Dwo embodimentsare disclosed, one having an air gap between the solid and annular core,while the other embodiment has a collapsible solid core.

DESCRIPTION OF THE DRAWING FIGURE 1 illustrates one embodiment of theinvention in an exploded view.

FIGURE 2 illustrates a second embodiment of the invention in an explodedview.

FIGURE 3 illustrates one form of a multiple mold casting apparatus.

FIGURE 4 illustrates a third embodiment of the invention in an explodedview.

DESCRIPTION OF THE INVENTION FIGURE 1 illustrates one form of theinvention for making annular fuel element in diameter by A" thick andapproximately 3" high.

An annular mold body or casing 1 may be of graphite or metal. The casingis closed at the bottom by a plug 2 having a circular recess 3.

The top of the casing is open and has a ridge 9 for supporting the plug4. The top plug 4 also includes a centrally located circular recess 5.The top and bottom plugs serve as support and centering plates for theannular hollow graphite core 6 and the solid graphite heat sink 7 whichfit within the recesses 3 and 5 as shown.

The top plug 4 also serves as a distributor of the liquid uraniumcarbide which is poured into the body of the mold casing to form theannular casting. The plug 4 is essentially a disc having four arms orsalient poles 8 which extend therefrom and are supported on the ridge 9of casing 1.

In the process of casting, molten uranium carbide is prepared in anargon atmosphere at 23 inches of mercury pressure, and is poured at 2500C.

The molten metal is poured onto and distributed by the distributor cap10 of the top plug 4; it flows in the voids between the arms 9 into theA" annular space 11 formed in the mold.

The thin wall elongated hollow graphite core 6 in all embodiments of theinvention is designed sufiiciently strong to restrain molten UC, but lowenough so that it will crack due to the contraction existing in the UCwhen cooled. In FIGURE 1, this tube is .010 inch thick and has a A inchdiameter.

An air gap 12 exists between the hollow core 6 and solid graphite core7. The air gap is quite small since the solid core 7 serves primarily asa heat sink to distribute the liquid metal heat by conduction to thecasing members 1, 2 and 4. The air gap space ideally is equal to thecontraction experienced by the fuel in cooling.

FIGURE 2 illustrates a second embodiment of the invention designed forcasting 2" diameter cores having the same /1" thickness as in FIGURE 1.Essentially, FIG- URE 2 is very similar to FIGURE 1 with severalexceptions. The same numerals are used as in FIGURE 1 for cor-respondingparts. The air gap 12 between hollow core 6 and solid core 7 islarger-of the order of .042 inch.

Because of the larger size of all the elements, the method of alignmentand centering of the solid core 7 is slightly different. The heat sinkaction of FIGURE 2 must be more efiicient than in FIGURE 1. Accordingly,the solid core 7 has a cap or extension 13 adapted for mating with theopening 14 in distributor plate 4. Thus the extension can radiate heatdirectly to the atmosphere as well as conductively through thedistributor. The bottom plate 2 includes. a ridge 15 for supportinghollow core 6 and spacing it from the solid core 7 which is supported onthe bottom of the plate.

FIGURE 3 illustrates a multiple mold casting method utilizing five moldsof the type in FIGURES l, 2 and 4. One of the problems encountered withmultiple molding is that the temperature of the liquid metal is usuallyhigher and there is a greater possibility of destroying the hollow coreduring casting. 1

Accordingly, a large conical graphite distributor 16 is used to providea laminar flow into the mold cavities. Liquid metal is poured from aladle or other means onto this distributor. A tube 17 is shown toillustrate the direction of pouring. This distributor device has provenquite effective in providing just sufiicient chilling of the molten UCto protect the cores. The use of a flat distributor caused too muchchilling and resulted in unacceptable mold.

FIGURE 4 illustrates a third embodiment of the invention which isparticularly suitable for large castings and other applications wheremore heat is generated then can be dissipated by the embodiments shownin FIGURES 1 and 2. This embodiment resembles those previously discussedin that it includes a mold cavity 11 defined by cylindrical outer casingor body 1, hollow collapsible core 6, and base plate 2; a top plate ordistributor 4; and an inner core or heat sink 7. It differs from FIGURES1 and 2 in that there is no air gap between hollow core 6 and heat sinkcore 7, the two cores being in direct contact with each other, therebyproviding more effective transfer of heat from the casting.

Heat sink core 7 comprises an assembly of three or more elongatedpie-shaped segments 27 linked together by resilient members or springsand shown separated by air gaps 28 in the normal or hot conditions. Forpurposes of illustration, four segments 27 are shown in FIGURE 4.Graphite spacer springs 20 tend to expand core assembly 7 radially, suchthat each of its segments 27 is pressed in direct contact against theinner surface of hollow core 6. At the same time, springs 20 areflexible enough to permit core assembly 7 to collapse along with hollowcore 6 as the casting cools and contracts, thereby closing the air gaps.In the drawing, springs 20 are shown as resilient C-rings operating inslots in the top and bottom of core segments 27.

The spring-loaded core assembly 7 in this embodiment tends to centeritself within core 6. Hence, additional centering means such as the pegand hole arrangement, 13 and 14, in FIGURE 2 is unnecessary. In FIGURE4, the opening 14 in top plate 2 is closed with plug 24.

Subsequently, as the liquid metal cools in annular space 11 and theannular fuel element contracts, the spacer rings 20 are compressed andcontract until limited by the meeting of the four segments in the centerof the mold with the elimination of air gaps 28.

The advantage of the apparatus of FIGURE 4 is that the solid core isalways in direct thermal contact with the hollow core and the liquidmetal, while yet permitting contraction of the fuel element. Thus thisembodiment is especially adapted for large fuel element casting.

The emodiment depicted in FIGURE 4 has produced crack-free uraniumcarbide fuel elements as large as 6 inches long and 3 /2 inches indiameter, with a 4-inch wall thickness, thus illustrating the merit ofcombining a flexible core with efiicient heat transfer means.

While the invention has been described as required,

the chart below indicates the relevant factors in design for severalsizes of molds:

TABLE l.DIMENSIONS OF SINGLE-OAVITY GRAPHITE MOLDS AND AS-CAST CYLINDERSCasting dimensions (in.) Shrinkage (AD) (in.)

Nominal A in. CD by Z in. ID by 3 in. long U0 casting Mold dimensions(in.)

Nominal 2.000 in. OD by 1.5 in. ID by 3 in. long UC casting Outermold-tube: OD: D.

D, 2. Top, 1.937 Top, 0.063 ID, 2.0 Bottom, 1.035 Bottom, -0.065 Innershell core: ID: D:

0D, 1.520 Top, 1.490 Top, 0.030

Bottom, 1.488 Solid core insert OD,

Bottom, 0.032

Nominal 3.500 in. OD by 3 in. ID by ll gin. long UO casting Outer moldtube: OD: D.

0D, 4.000 Top, 3.305 Top, 0.105 ID, 3.500 Bottom, 3.390 Bottom, 0.l10Inner shell core: ID: I ID:

3.000 Top, 2.030 Top, 0.070 ID, 2.970 Bottom, 2.030 Bottom, 0.070

Solid core insert OD,

TABLE 2.DIAMETRICAL SIIRINKAGE OF CAST ANNULAR UC FUEL [Measured vs.calculated] Diametrieal shrinkage (in.)

Nominal as-east Measured (avg) Calculated cylinder OD (in.) O D ID 0 DID While several embodiments of the invention have been described, itwill be obvious that modification can be made thereto. Furthermore, theinvention has been de scribed with reference to uranium carbide andother brittle nuclear fuel elements. However, it is anticipated that theinvention may find use in other and less critical cast ing applications.Accordingly, the invention is defined in the following claims.

What is claimed is:

1. Apparatus for casting annular elongated fuel ele ments comprising: anelongated hollow casing having an open end and a closed end; a thin,elongated annular hollow graphite core and a second graphite core withinand coextensive with said hollow core; and a graphite distribution platehaving a plurality of salient poles posi' tioned for substantiallyclosing the open end of said casing and for positioning both cores withtheir axes coaxial with said casing, whereby a cavity is formed betweenthe inside diameter of the casing and the outer diameter of the hollowcore, said second core including a plurality of elongated coextensivespaced segments of solid graphite and a resilient graphite ring-typespring adapted for permitting expansion and compression of said segmentsin unison in intimate contact with the inner circumference of the hollowcore while the poured liquid metal cools.

2. Apparatus for casting annular elongated fuel elements comprising: anelongated hollow casing having an open end and a closed end; a thin,elongated annular hollow graphite core and a second graphite core withinand coextensive with said hollow core; and a graphite distribution platehaving a plurality of salient poles positioned for substantially closingthe open end of said casing and for positioning both cores with theiraxes coaxial with said casing, whereby a cavity is formed between theinside diameter of the casing. and the outer diameter of the hollowcore, said second core including a plurality of elongated coextensivespaced segments of solid graphite 5 6 and a resilient graphite ring-typespring adapted for perhollow core as metal poured into the mold cavitymitting expansion and compression of said segments in cools; and unisonin intimate contact with the inner circumference (d) a distributor platelocated at the open end of the of the hollow core while the pouredliquid metal cools, casing and cavity for centering the cores withinsaid segments being spaced from each other a distance the casing and forsealing oil the volume within corresponding to the contraction of liquidmetal in the 5 the hollow core from the flow of metal poured into cavitywhen cooled. the mold cavity.

3. Apparatus for casting elongated annular fuel elez p h n h dReferences Cited a an eongate o ow casing aving an open en and a dog e dend; 10 UNITED STATES PATENTS (b) a thin, elongated hollow corepositioned within 773,590 11/1904 Rafter 2 said casing such that anelongated annular mold 1,946,451 2/ 1934 l y 9 X cavity is formed havingone open end and one 7 K 1/1957 BYOWHStBiH- closed end between thecasing and the hollow core; 15 2,405,254 8/1946 H p in 2 2 X (c) aflexible heat sink core assembly within the hol- 2,583,248 1/1952Bajfovic 249-42 X low core, comprising a plurality of elongatedpieshaped segments linked together by resilient means, ROBERT D.BALDWIN, Primary Examiner for maintaining the core segments in intimatecon- US Cl XR tact with the inner surface of the hollow core, and 20 forpermitting the assembly to contract with the 249-107, 122, 134, 177

